Key points:

City with 200,000 people remains under siege, with food supplies reportedly running low

At least 120 government forces and 70 IS fighters killed in clashes

UN says thousands have been put into 'the firing line'

The Syrian Observatory for Human Rights said IS militants freed the group — which included women, children under 14, and the elderly — after questioning them and concluding they had no ties to Syria's regime.

The jihadists have tightened their siege on the city and residents have been left to fear the worst.

Fierce clashes rocked the city's northern edges after a multi-front offensive on Saturday that cost the lives of dozens of people.

The extremists now control 60 per cent of Deir Ezzor, while the city's roughly 200,000 residents face fear and hardship

According to the United Nations, about 70 per cent of the city's remaining residents are women and children.

Al-Baghaliyeh, one of the last agricultural areas on the city's outskirts, has been overrun by the jihadists, the AFP news agency reported.

"People are afraid, the situation is very difficult," Attiyeh, a resident of Deir Ezzor, said by phone from the city.

"Food and vegetables are rare, and we're starting to have problems with bread," he added.

He said reports circulated by state media that IS fighters had killed 300 civilians in the city had sparked even more fear.

"If the city falls there will be a massacre," Attiyeh said.

Thousands in the line of fire: UN

The attack and subsequent fighting has killed at least 120 members of government forces, many of whom were executed by IS, according to the Syrian Observatory for Human Rights.

The group also said at least 70 IS fighters had been killed in the assault and clashes, with the jihadists seizing territory inside and outside of the city's north-western tip.

On Monday, IS consolidated its gains on the city's edges, taking advantage of a dust storm that all but grounded Russian warplanes that had been carrying out strikes in support of regime troops, according to Observatory chief Rami Abdel Rahman.

Since early 2015, IS has blocked off access to most of the city, leading to "a severe shortage of food, medicine, and basic commodities," according to the United Nations.

Its most recent offensive "is putting thousands of people in the line of fire," said Linda Tom, spokeswoman for the UN's Office for the Coordination of Humanitarian Affairs, or OCHA.

"The UN has received credible reports of the execution and abduction/detention of civilians, including people believed to have been smuggling food into the besieged city," she told AFP.

Syria's Government airlifted basic supplies into the city on January 11. The Russian Government said a few days later it had also dropped aid into besieged neighbourhoods.